If it is ASTM A53, Type S means seamless.
Type F is furnace but welded,
Type E is Electrical resist welded.
Thats how. It is the easiest way to identify wether pipe is seamless or ERW.
In short, Seamless is marketed on an outdated myth that it does not have flaws but that welded has an inherent flaw throughout the length of each tube, as stated above. Intuitively this concept is easily accepted by purchasers and designers with limited experience with the various products. Each product form and manufacturing method has its own inherent problems and potential defects.
Consumers must become familiar with what those problems and defects are, and how they might affect particular applications. Then they can make an informed decision based on facts, rather than myth.